Antero-posterior stabilization of the acromioclavicular joint (AC-joint) is generally maintained via the ligamentum acromioclaviculare and parts of the trapezius and deltoideus muscle. Cranial stabilization is generally maintained via the conoid ligament and the trapezoid ligament (between the coracoid and the clavicle). A fall onto the shoulder with a direct force to the clavicle or on the outstretched arm is a common cause for fractures of the bone or ruptures of the stabilizing soft tissues. Simple fractures or minor AC luxations are treated conservatively. However, more complex fractures such as, for example, Neer type II, Jäger and Breitner type II and acromio-clavicular joint dislocation types such as Tossy III and Rockwood III to V are treated operatively. The Neer and the Jäger and Breitner types involve a bone fracture while the Tossy and Rockwood types involve ruptures of the ligaments without bone fractures.
Various types of fixation techniques and procedures exist for the treatment of these dislocations, which may be divided into four basic principles:
1. Acromio-clavicular repairs (Fixation between the acromion and the distal clavicle);
2. Coraco-clavicular repairs (Fixation between the coracoid and the distal clavicle);
3. Distal clavicle excision; and
4. Dynamic muscle transfers.
Acromio-clavicular fixation may be performed using a clavicle hook plate and variations thereof such as, for example, the Balser plate and the Wolter plate. Currently, the hook plate is used to treat clavicle fractures associated with ruptures of the conoid ligament and trapezoid ligament. The hook plate may be configured in either a left or right version and is commonly formed with three different hook depths. The plate is used to join two bone fragments while the hook is placed under the acromion and used to pull the clavicle to a normal position generally maintained by the ligaments.
However, many patients treated with a hook plate show a phenomenon called “hook migration” in which the hook moves in a cranial direction during healing. When the hook moves in a cranial direction, the hook penetrates the subacrominal roof causing osteolysis.
Currently, the only solution to stop erosion and prevent osteolysis is to remove the osteosynthesis device early and hope that the bone fragments have consolidated. Thus, a major drawback of existing hook plates is the occurrence of osteolysis because of the hook being pressed against the AC-roof, which may lead to early removal of the plate even when the fracture has not healed completely. A hook plate with a larger hook would result in subacrominal impingement.
As discussed above, a misplaced hook will begin to erode the acromion. In some situations, even a correctly placed hook may eventually result in erosion. In these situations, the clavicle hook plate must be removed after 3 months. After approximately three months the ruptured ligaments have healed but the fracture may still be unstable. Thus, the surgeon may have to make the difficult decision of either leaving the plate inside the patient and risking further damage to the acromion or removing the plate early to prevent harm to the acromion while risking a new fracture.